I love them both! They both did first-rate work and, apparently, were also very nice people. In a way they were opposites--Katherine was boyish and strong-willed, Audrey was beautiful and vulnerable and feminine.
It's not easy to choose between them but I guess I'd pick Katherine. They were both excellent in their roles but the movies Katherine made couldn't have been done by anyone else. The same is true of Audrey, that nobody really could replace her. But I think it's even more true of Katherine. Her character was really unique.
We saw a remake of Sabrina a few years ago. The original is one of my favorite Hepburn movies, the new one starred Julia Ormond, who was okay but not as good. So yeah, Ormond couldn't replace Audrey. But have you ever seen a remake of a Katherine Hepburn movie? They would never even try!